1. Field of Invention
The present invention relates to a howling suppression device, a howling suppression program, an integrated circuit, and a howling suppression method. More particularly, the present invention relates to a howling suppression device, a howling suppression program, an integrated circuit, and a howling suppression method for suppressing the occurrence of howling in a sound-intensifying system for intensifying, through a loudspeaker, a sound signal collected by a microphone.
2. Description of the Related Art
In the prior art, howling suppression devices have been developed for suppressing the occurrence of howling in a sound-intensifying system for intensifying, through a loudspeaker, a sound signal collected by a microphone. A conventional howling suppression device employs a method using the amplitude control of a narrow-band signal (e.g., a notch filter, or a graphic equalizer) for suppressing the signal amplification factor at a frequency at which howling occurs. The method for the amplitude control may be a semi-static method in which the adjustment is done at installation, a method in which a howling detection section is provided for dynamic control based on the detection results, etc., (see, for example, Patent Document 1 and Patent Document 2).
FIG. 7 is a block diagram showing a configuration of a sound-intensifying device disclosed in Patent Document 1. In FIG. 7, the sound-intensifying device includes a microphone 101, a loudspeaker 103, a howling detection section 104, an amplitude-frequency characteristics correcting section 105 and a signal amplification section 106.
Next, the operation of the conventional sound-intensifying device will be described. In the sound-intensifying device, a sound signal received from the microphone 101 is input to the amplitude-frequency characteristics correcting section 105, and the amplitude-frequency characteristics correcting section 105 corrects the frequency characteristics. The amplitude-frequency characteristics correcting section 105 outputs the corrected sound signal to the signal amplification section 106. Then, the signal amplification section 106 amplifies the received sound signal, and a sound based on the sound signal is output from the loudspeaker 103 into the sound field.
Howling occurs at a frequency at which the gain of the loop of the transmission system exceeds one due to the intensified sound from the loudspeaker 103 being introduced back into the microphone 101. Therefore, in order to suppress the howling while keeping the sound intensification level, the signal level is attenuated only for a frequency band where the loop gain exceeds one. The frequency band to be attenuated is pre-adjusted according to the sound field in which the sound-intensifying device is installed. The environment of the sound field varies depending on the position of the microphone 101 during the use of the sound-intensifying device. Therefore, the occurrence of howling is detected by the howling detection section 104 to constantly control the frequency band to be attenuated by the amplitude-frequency characteristics correcting section 105, thereby realizing a more versatile sound-intensifying device.
FIG. 8 is a block diagram showing a configuration of a howling cancellation device disclosed in Patent Document 2. In FIG. 8, the howling cancellation device includes the microphone 101, the loudspeaker 103, a signal subtraction section 107, an adaptive filter section 108, and a signal amplification section 109.
Next, the operation of the conventional howling cancellation device will be described. In the howling cancellation device, the sound signal received from the microphone 101 is input to the signal subtraction section 107, and the signal subtraction section 107 performs a subtraction operation between the sound signal and the output signal from the adaptive filter section 108. The signal subtraction section 107 outputs the subtracted output signal to the signal amplification section 109. Then, the signal amplification section 106 amplifies the received output signal, and a sound based on the sound signal is output from the loudspeaker 103 into the sound field. Based on the output signal from the signal amplification section 109 and the output signal from the signal subtraction section 107, the adaptive filter section 108 estimates the transmission characteristics of the sound field through which the intensified sound output from the loudspeaker 103 enters the microphone 101 (the transmission characteristics of the loudspeaker 103 and the transmission characteristics of the microphone 101), and outputs the pseudo echo of the intensified sound coming from the loudspeaker 103 and entering the microphone 101 to the signal subtraction section 107. Thus, in the signal subtraction section 107, a component of the intensified sound from the loudspeaker 103 that travels around back to the microphone 101 is canceled with the pseudo echo produced by the adaptive filter section 108, thereby cutting off the howling loop, providing a howling suppression effect.
Patent Document 1: Japanese Patent No. 3152160
Patent Document 2: Japanese Patent No. 2560923